One example of multiple access protocols whereby a number of communication apparatus mutually performs communication is CDMA (Code Division Multiple Access) that uses the spread spectrum technology. CDMA excels in terms of privacy protection and interference resistance, and, with frequency efficiency that is high, is capable of accommodating a large number of users. The CDMA protocol has a history of standardization upon the basis of speech data transmission, and the transmission speed per channel assigned to each communication terminal is set at an adequate speed for speech data transmission. The IS-2000 system provides services for transmitting at relatively low rates data which requires to be transmitted in real time (in other words, which does not permit delay) such as speeches, and data which does not require to be transmitted in real time such as low-rate packets.
However, a need for transmitting large-volume data on forward link channels has grown with the development of services in variety in recent years. To meet this need, HDR (High Data Rate) has been proposed as a technique to enhance transmission rates on forward link channels from base stations to communication terminals. A communication system with HDR (hereinafter “HDR system”) uses a wireless transmission scheme that does not require transmit power control, and, by means of using the same frequency bandwidths as those of IS-2000 system (1.25 MHz band) on both reverse link and forward link channels and performing transmission at a constant transmit power on forward link channels, implements the services of high-speed packet communication in the same covered area with IS-2000 system.
Generally, a wireless channel for high-speed packets has high symbol rates and requires greater transmit power than a wireless channel with relatively low symbol rates. Maintaining the wireless channels for high-speed packets therefore requires considerably great transmit power, and as a result, wireless channels for high-speed packets become great interference and a cause of a decrease in the system capacity.
To solve this problem, HDR system assigns communication resources in accordance with the channel quality while performing transmission at a constant transmit power on forward link channels. As shown in FIG. 1, communication is performed with each user at a constant transmit power, while the communication mode including the slot length, coding rate, modulation scheme, and spreading factor vary corresponding to the channel quality of individual users. As one way of assigning communication resources like increasing transmission rates with users with a good-quality channel, the enhancement of the overall system by means of controlling the slot length, coding rate, modulation scheme, and spreading factor is presently under consideration. In FIG. 2, channel quality is good with user 2, and a large number of slots are assigned accordingly to the data for user 2.
High-speed packet communication performed between base stations and communication terminals in HDR system will be described now. FIG. 2 shows a sample configuration of a transmit packet used in HDR system. As shown in FIG. 2, a packet used in HDR system is comprised of slots each with data portion 41 on which data for users are time-multiplexed and with header 42 containing a pilot signal and control information added before data portion 41. The control information contains assignment information that denotes the assignment of communication resources for each communication terminal.
First, a base station transmits a packet configured as shown in FIG. 2 to each communication terminal in its own covered area. Each communication terminal measures the forward link channel quality (CIR (Carrier to Interference Ratio), for instance) based on the pilot signal contained in a received signal. Each communication terminal stores a table that denotes correspondence between forward link channel qualities and the best communication schemes for packet transmission with these channel qualities. Each communication terminal refers to this table and selects a communication scheme that enables the most efficient high-speed packet communication with the measured channel quality. A communication mode refers to a combination of the slot length assigned to transmit data, coding rate, modulation scheme, and spreading factor for transmit data. Each communication terminal transmits signals (Data Rate Control Signals: DRC signal) that denote selected communication scheme to the base station. Other communication terminals accommodated under the same base station similarly transmit DRC signals to the base station.
A base station refers to the DRC signals transmitted from each communication terminal and assigns communication resources, whereupon priority is given to communication terminals with good channel quality. By this means, data is transmitted at high transmission rates to communication terminals with good channel quality so that it is possible to reduce the time required for communication, while data is transmitted at low transmission rates to communication terminals with poor channel quality so that it is possible to improve error resistance. Incidentally, determining the assignment of transmit slots in accordance with the channel quality at a base station under HDR system is called “scheduling.”
A base station assigns slots to transmit data in accordance with the communication resource assignment and performs decoding processing, modulation processing, and spreading processing with respect to the transmit data. A transmit frame is configured when transmit data for each communication terminal is time-division-multiplexed after the above processing. A transmit frame is transmitted to each communication terminal, whereupon the control information (assignment information) that denotes the assignment of communication resources with each communication terminal is inserted into a header at ahead of a transmit frame. A communication terminal learns the communication scheme by referring to the assignment information and is able to demodulate the data for the communication terminal.
The conventional HDR system enhances data transmission efficiency of the overall system by thus prioritizing communication terminals with good channel quality and by assigning communication resources accordingly.
In recent years, however, the construction of a communication system that combines HDR system and IS-2000 system by means of assigning frequencies that are different from those of IS-2000 system to the above-mentioned HDR system has been proposed. Services that this communication system combining HDR system and IS-2000 system offers (abbreviated “1xHDR/IS-2000”) combine the high-speed packet communication services offered by HDR system and the speech communication services offered by IS-2000 system. With 1xHDR/IS-2000, HDR system and IS-2000 system can be switched at the user's choice using one communication terminal, whereby a variety of services can be provided.
However, HDR system and IS-2000 system are communication systems that are originally constructed upon different basis. When it comes to the services (1xHDR/IS-2000) that a communication system combining these systems offer, a problem arises that the consistency between the services of these systems may be incomplete and therefore both systems cannot operate efficiently. More specifically, when a communication terminal performing high-speed packet communication in HDR system receives a call under IS-2000 system, and when communication under IS-2000 system thus interrupts high-speed packet communication under HDR system, the following problems arise:    (1) When an interruption occurs and high-speed packet communication under HDR system is prioritized, the calling user has no choice but wait after a call from IS-2000 system until high-speed packet communication under HDR system is finished and communication starts under IS-2000 system, which deteriorates the transmission efficiency of the overall system. Furthermore, the communication terminal the calling user uses continues calling constantly even during the waiting period, and the power consumption of the communication terminal increases.    (2) When there is an interruption and high-speed packet communication is suspended before completion and communication under IS-2000 system is prioritized, the data in high-speed packet communication needs to be transmitted again, which consumes communication resources and increases the power consumption.